Spur mitigation for wireless communication systems

ABSTRACT

A receiver for a wireless communication device comprising a mechanism for performing spur mitigation. A spur mitigation unit of the receiver is operable to determine a current estimate of a spur signal from an ADC output signal using a growing box filter. The spur mitigation unit can determine a power of the current estimate of the spur signal and a power of a previous estimate of the spur signal using a power measurement unit. The spur mitigation unit can determine whether the power of the previous estimate of the spur signal is greater than the power of the current estimate of the spur signal. If the power of the previous spur signal estimate is greater than the power of the current spur signal estimate, the spur mitigation unit can remove the spur signal from the ADC output signal based on the previous spur signal estimate.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/335,832 filed on Dec. 16, 2008.

BACKGROUND

Embodiments of the inventive subject matter generally relate to thefield of wireless communication networks, and more particularly, totechniques for spur mitigation.

Wireless communication systems can use one or more channels to transferdata between a transmitter and receivers. These communication systemscan operate according to a set of standards, defined by the Institute ofElectrical and Electronics Engineers (IEEE) 802.11 committee, forWireless Local Area Network (WLAN) communication. 802.11b is a part ofthe IEEE 802.11 specification and defines transmission standards toincrease network throughput and support bandwidths of up to 11 Mbps, byusing the 2.4 GHz frequency spectrum.

SUMMARY

Various embodiments are disclosed of a receiver for a wirelesscommunication device comprising a mechanism for performing spurmitigation. In one embodiment, the receiver comprises ananalog-to-digital converter (ADC) and a spur mitigation unit. The spurmitigation unit comprises a growing box filter and a power measurementunit. The ADC is operable to convert an RF signal received at thereceiver to a digital signal. The spur mitigation unit is operable toreceive an output signal of the ADC. The growing box filter of the spurmitigation unit is operable to determine a current estimate of a spursignal of the ADC output signal. The power measurement unit of the spurmitigation unit is operable to determine a power of the current estimateof the spur signal and a power of a previous estimate of the spursignal. The power measurement unit is also operable to determine whetherthe power of the previous estimate of the spur signal is greater thanthe power of the current estimate of the spur signal. If the power ofthe previous spur signal estimate is greater than the power of thecurrent spur signal estimate, the spur mitigation unit is operable toremove the spur signal from the ADC output signal based on the previousspur signal estimate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments may be better understood, and numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 is an example block diagram of a receiver unit comprisingmechanisms for spur mitigation;

FIG. 2 is a flow diagram illustrating example operations for spurmitigation in data processing;

FIG. 3 is an example conceptual diagram illustrating the effect of thesize of the growing box filter and accuracy of the spur estimate on spurmitigation;

FIG. 4 is a flow diagram illustrating example operations for AGC controlof spur mitigation in packet detection;

FIG. 5 is a flow diagram illustrating example operations for spurmitigation in packet detection; and

FIG. 6 is an example wireless device including a mechanism forimplementing spur mitigation.

DESCRIPTION OF EMBODIMENT(S)

The description that follows includes exemplary systems, methods,techniques, instruction sequences, and computer program products thatembody techniques of the present inventive subject matter. However, itis understood that the described embodiments may be practiced withoutthese specific details. In some instances, well-known instructioninstances, protocols, structures, and techniques have not been shown indetail in order not to obfuscate the description.

A transmitted radio frequency (RF) signal typically travels by multiplepaths (e.g., line of sight, reflecting off one or more objects in itspath, etc.) as it passes through a communication channel (e.g., freespace). The transmitted signal may be corrupted by noise, interfere withother signals, and get attenuated before it is received by a receiverunit comprising one or more receiver chains. Transmitters that aredesigned to transmit RF signals using IEEE 802.11b standards use acomplementary code keying (CCK) modulation technique defined by the802.11b standards to support data rates of up to 11 Mbps in the 2.4 GHzfrequency band. CCK is a variation of direct sequence spread spectrumtechniques. A CCK modulator uses a pre-defined set of complexexpressions and derives a different spreading code, depending on theinformation bits to be transmitted. CCK is typically used in 802.11bstandards to increase the data rate of the transmitted signal.

At the receiver, to detect incoming RF signals, receiver units(comprising one or more receiver chains) may use an autocorrelation unitto detect the presence of a periodic RF signal with a pre-definedperiodicity (e.g., 1 microsecond). However, the auto-correlation unitis, very often, unable to distinguish between a periodic noise signaland a desired RF signal because the auto-correlation unit can detect anyperiodic signal with the desired RF signal's periodicity. In otherwords, the auto-correlation unit can detect any signal with aperiodicity of 1 microsecond or multiples of 1 microsecond. Digitalclocks present on the transmitter and receiver can have harmonics withthe same periodicity as the received RF signal. The autocorrelation unitmay detect the harmonics of the digital clock and trigger false packetdetection. These harmonics of the digital clock and other periodic noisesignals that lead to false detection triggered by the autocorrelationunit are spurs. Improper detection can cause inaccuracies in timing andfrequency offset estimation, incorrect decoding, and result in adegradation of receiver performance.

Spur mitigation in packet and data processing paths can help ensure thatthe receiver unit detects the appropriate received signal and not aperiodic spur signal. Furthermore, dynamically configuring the size offilters used for spur mitigation can improve spur estimates and receiverperformance. Also, the spur's power may be larger than the in-band noisepower, sometimes by a factor of 10 dB. Scaling the received RF signalbefore and after spur mitigation to fit the available bit-width andsignal levels can ensure optimal representation of the RF signal, fewerdecoding errors, and better receiver performance.

FIG. 1 is an example block diagram of a receiver unit comprisingmechanisms for spur mitigation. The receiver unit 100 comprises anantenna 102, a variable gain amplifier (VGA) 104, an analog to digitalconverter (ADC) 106, one or more filter stages 110, and a root raisedcosine (RRC) filter 112. The output of the ADC 106 is also fed to anautomatic gain control (AGC) unit 108, which in turn, controls the VGA104.

The output of the RRC filter 112 is coupled with a spur mitigation unitfor packet detection 160 and a spur mitigation unit for data processing170. In the spur mitigation unit for packet detection 160, the output ofthe RRC filter is multiplied, using multiplier 114, with frequency F1. Amoving average filter 116 uses the output of the multiplier 114 todetermine a spur amplitude estimate in a preamble portion of thereceived signal. Additionally, the AGC unit 108 is also coupled with themoving average filter 116. Multiplier 118 then multiplies the output ofthe moving average filter 116 with frequency F2, thus shifting theestimated spur back to its spur frequency. Adder 120 subtracts the spur(output of the multiplier 118) from the signal (output of the RRC filter112). Subsequent packet processing units 122 further process the outputof the adder 120.

In the spur mitigation unit for data processing 170, the output of theRRC filter is multiplied, using multiplier 130, with frequency F1. Agrowing box filter 132 uses the output of the multiplier 130 todetermine a spur amplitude estimate in a data portion of the receivedsignal. Multiplier 136 then multiplies the output of the growing boxfilter 132 with frequency F2, thus shifting the estimated spur back toits spur frequency. Additionally, a power measurement unit 134 alsoreceives the output of the growing box filter 132, computes the power ofthe estimated spur, and controls the length of the growing box filter132. Adder 138 subtracts the spur (output of the multiplier 136) fromthe signal (output of the RRC filter 112). A digital scalar (142) scalesthe output of the adder 138 based on a power measurement from powermeasurement unit 140. Subsequent baseband processing units 122 furtherprocess the output of the digital scalar 142.

The antenna 102 receives RF signals and provides the received RF signalsto the variable gain amplifier (VGA) 104. The VGA typically varies itsgain depending on a control signal (e.g., control voltage) applied. Theautomatic gain control (AGC) unit 108 provides this control voltage andvaries the control voltage depending on the amplitude of the received RFsignal and the maximum bit-width of the ADC 106. The AGC 108 directs theVGA 104 to increase or decrease the applied gain such that the receivedRF signal (including the spur) optimally fits the ADC's availablebit-width. After the ADC quantizes the received RF signal and converts acontinuous analog signal into a discrete digital signal, the digitizedsignal goes through one or more filter stages 110. In someimplementations, a Fast Fourier Transform (FFT) unit may convert thetime domain signal into a frequency domain signal. The root raisedcosine filter 112 is a pulse shaping matched filter that can be used in802.11b circuitry to minimize inter-symbol interference (ISI) and theeffects of noise on the received signal. A root raised cosine filterwith the same response (as RRC filter 112) is also used at thetransmitter to shape the response of the transmitted signal. At thereceiver, the RRC filter 112 has the same response as the RRC filter atthe transmitter. The RRC filter 112, therefore, filters the receivedsignal such that the filtered signal has the same response as thetransmitted signal. Also, the overall filter response, taking intoaccount the transmitter, channel, and receiver, is that of a raisedcosine filter, which can result in received signals with little or noISI. Therefore, using a matched RRC filter may help ensure a received RFsignal with maximum SNR (signal to noise ratio) and minimum ISI. In someimplementations, a matched RRC filter may not be used at the transmitterand the receiver. The transmitter and the receiver may comprise anysuitable filters to obtain the desired frequency response.

The output of the RRC filter 112 comprises one or more spur signals.Also, any received RF signal comprises a preamble and a payload. Thepreamble comprises training symbols, which can be used for channelestimation, frequency offset correction, timing offset correction, etc.,and the payload comprises data symbols. The preamble and the payload canbe processed separately. As shown in FIG. 1, the spur mitigation unitfor packet detection 160 processes the preamble while the spurmitigation unit for data processing 170 processes the payload.

In the spur mitigation unit for packet detection 160, multiplier 114shifts the received RF signal to DC (0 Hz). In FIG. 1, F1 represents aconjugate of a phase slope at the spur frequency (f_(spur)) which isrepresented as e^(−j2πf) ^(spur) ^(t). The moving average filter 116processes the shifted RF signal at the output of the multiplier 114 andfilters out part of the RF signal that is not the spur. The output ofthe moving average filter 116 is an estimate of the spur's amplitude andphase. Multiplier 118 shifts the spur estimate back to its original spurfrequency (represented by F2) by multiplying the spur estimate bye^(j2πf) ^(spurt) ^(t). The adder 120 subtracts the shifted spurestimate (at the output of the multiplier 118) from the received RFsignal (at the output of the RRC filter 112). Subsequent packetprocessing units 122 process the RF signal after the spur is removed.Subsequent packet processing units can comprise a CCK demodulator, atiming and frequency correction unit, a channel estimation unit, etc.

In the spur mitigation unit for data processing 170, multiplier 130shifts the received data signal to DC (0 Hz). As described earlier, F1represents a conjugate of the phase slope at the spur frequency(f_(spur)) which is represented as e^(−j2πf) ^(spurt) ^(t). The growingbox filter 132 processes the shifted data signal at the output of themultiplier 130 and filters out a part of the data signal that is not thespur. The growing box filter 132 progressively processes an increasingnumber of samples of the data signal. The output of the growing boxfilter 132 is an estimate of the spur. The power measurement unit 134determines the power of the current and previous spur estimates,compares the two measurements, and determines whether the power of theprevious spur estimate is greater than the power of the current spurestimate. The power measurement unit 134 controls the length of thegrowing box filter 132. Multiplier 136 shifts the spur estimate back toits original spur frequency (represented by F2) by multiplying the spurestimate by e^(j2πf) ^(spurt) ^(t). The adder 138 subtracts the shiftedspur estimate (at the output of the multiplier 136) from the received RFsignal (at the output of the RRC filter 112). Power measurement unit 140computes the power of the RF signal (after the spur is removed) and thedigital scalar 142 scales the RF signal, to better fit the availablebit-width, based on the computed RF signal power. Subsequent basebandprocessing units 144 can comprise a CCK demodulator, a deinterleaver, adecoder, and other digital components depending on the encoding appliedbefore transmission.

Although, FIG. 1 is depicted as comprising only one receiver chain, thereceiver unit 100 can include any suitable number of receiver chains.The outputs of the multiple receiver chains can be combined usingmaximum ratio combining techniques (MRC) to combine the outputs fromvarious receiver chains, to determine information symbols.

FIG. 2 is a flow diagram illustrating example operations for spurmitigation in data processing. The flow 200 begins at block 202.

At block 202, a data signal is received by a spur mitigation unit fordata processing, e.g., spur mitigation unit for data processing 170 ofFIG. 1. A received signal typically comprises two parts—a preamble and apayload. The preamble comprises training symbols, which are used inpacket detection, while the payload comprises data symbols. In someimplementations, the data signal can refer to the payload portion of thereceived signal. The data signal may be acquired from an ADC unit, oneor more filter stages, or any suitable processing unit. In someimplementations, an AGC unit may process the data signal before thesignal is provided to the spur mitigation unit for data processing. Theflow continues at block 204.

At block 204, a spur in the data signal is shifted to DC (0 Hz). Spurstypically occur at harmonics of a digital clock frequency. In someinstances, a frequency generator (e.g., local oscillator) may generatethe spurs. Shifting the spur frequency to DC (0 Hz) allows for easierestimation of spur amplitude. Shifting the spur frequency to DC alsoallows a low-pass filter to filter out all parts of the data signal thatare not the spur. The data signal comprises the spur at the spurfrequency. The spur in the data signal can be shifted to DC bymultiplying the data signal by a conjugate of the spur frequency, e.g.,via multiplier 130. Eq. 1 represents the operation of shifting the spurin the received data signal to DC.

$\begin{matrix}\left. \begin{matrix}{y = {\left( {h*x} \right) + n + {A\;{\mathbb{e}}^{j{\lbrack{{({2\pi\; f_{{spur}^{t}}})} + \varphi}\rbrack}}}}} \\{{y \cdot {\mathbb{e}}^{{- {j2\pi}}\; f_{{spur}^{t}}}} = {{\left\lbrack {\left( {h*x} \right) + n} \right\rbrack \cdot {\mathbb{e}}^{{- {j2}}\;\pi\; f_{{spur}^{t}}}} + {A\;{\mathbb{e}}^{j\;\varphi}}}}\end{matrix} \right\} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

In Eq. 1, y, h, x, and n respectively represent the data signal, channelestimates, transmitted signal, and additive noise. The spur amplitude,frequency, and phase are respectively represented by A, f_(spur), and φ.After this operation, the spur (represented by Ae^(jφ)) is at DC (i.e.,has no frequency component) and comprises an amplitude and phasecomponent. The flow continues at block 206.

At block 206, a spur estimate is calculated using a growing box filter.The growing box filter can be considered to be an expandable low passfilter. The growing box filter can comprise an accumulator thatprogressively accumulates and processes an increasing number of samplesas symbols from the data signal are received. The growing box filter candetermine the spur estimate by calculating an average of samples of thereceived data symbols. The flow continues at block 208.

At block 208, a power of the spur estimate is determined. For example,the power measurement unit 134 can determine the power of the spurestimate. The power of the spur estimate can be used to determinewhether the growing box filter has accurately estimated the spuramplitude. In other words, by calculating the power of the spurestimate, it can be determined whether the growing box filter hasremoved part of the spur along with the data signal. As describedearlier, the spur comprises an amplitude and phase component and isrepresented by Ae^(jφ). The spur estimate is a complex quantity andtherefore, the power of the spur estimate (at the output of the growingbox filter) can be described using Eq. 2. In Eq. 2, P_(spur),Im(P_(spur)), and Real(P_(spur)) represent the power of the spurestimate, imaginary component of the spur estimate, and real componentof the spur estimate respectively. The flow continues at block 210.P _(spur)=√{square root over ((Real(P _(spur)))²+(Im(P_(spur))))}{square root over ((Real(P _(spur)))²+(Im(P _(spur))))}  Eq.2

At block 210, a power of a previous spur estimate is calculated. Forexample, the power measurement unit 134 can determine the power of theprevious spur estimate. The power of the previous spur estimate from thegrowing box filter can be calculated using Eq. 2. In someimplementations, the previous spur estimate from the growing box filtermay be stored (either within the growing box filter or separately fromthe filter). The power of the previous spur estimate may be computedafter a new spur estimate is determined. The flow continues at block212.

At block 212, it is determined whether the power of the previous spurestimate is greater than the power of the current spur estimate. Thegrowing box filter removes, through a series of iterations, componentsof the signal that are not the spur. A change (an increase or adecrease) in spur power can indicate that the growing box filterfiltered out part of the spur along with the data signal. This change inthe spur power can indicate that the spur frequency estimate isinaccurate and that the growing box filter is not centered on the spurfrequency. In some implementations, it may be determined whether thedifference between the power of the current and previous spur estimatesis greater than a pre-defined threshold. In other implementations, itmay be determined whether the power of the current spur estimate iswithin a certain percentage of the previous spur estimate. For example,if it is determined that the power of the current spur estimate iswithin 90% to 110% of the power of the previous spur estimate, thecurrent spur estimate may be assumed to be an accurate estimate. Inother words, if the difference between the power of the previous spurestimate and the current spur estimate is within 10% of the power of theprevious spur estimate, the current spur estimate may be assumed to bean accurate estimate. If it is determined that the power of the previousspur estimate is greater than that of the current spur estimate, theflow continues at block 214. Otherwise, the flow continues at block 220.

At block 214, the previous spur estimate is shifted to the original spurfrequency, e.g., via multiplier 136. A change (an increase or adecrease) in spur power can indicate that the growing box filterfiltered out part of the spur along with the data signal. The previousspur estimate can be considered to be a better estimate of the spur.Therefore, the previous spur estimate is used in further spur mitigationoperations. At block 204, the spur was shifted to DC to enable thegrowing box filter to determine a best estimate of the spur amplitude(and phase). To remove the spur from the data signal, the spur estimateshould be accurately represented in the frequency domain and therefore,should be shifted to its original spur frequency. This is done, asdepicted in Eq. 3, by multiplying the spur estimate from the growing boxfilter (Ae^(jφ)) by a phase slope representing the spur frequencyf_(spur) (i.e., e^(j2πf) ^(spur) ^(t)). In Eq. 3, the shifted spurestimate is represented by y_(spur). The flow continues at block 216.y _(spur) =Ae ^(jφ) ·e ^(j2δf) ^(spurt) ^(t) =A·e ^((jφ+j2πf) ^(spurt)^(t))  Eq.3

At block 216, the shifted spur estimate (determined at block 214) issubtracted from the acquired data signal (received at block 202), e.g.,via adder 138. This operation removes the spur from the data signal(depicted in Eq. 4). In Eq. 4, y_(data), y_(spur), and y_(compensated)represent the received data signal, the shifted spur estimate, and thecompensated data signal after spur removal respectively. The flowcontinues at block 218.y _(compensated) =y _(data) −y _(spur)  Eq. 4

At block 218, the compensated data signal is provided to a digitalscalar. After the spur is removed from the data signal, the strength ofthe resultant signal may be too small to be accurately represented bythe original received signal quantization (performed by the ADC beforespur removal). The digital scalar can be used to scale the compensatedsignal (after spur removal) to better fit the available bit-width. Insome implementations, digital scaling may be controlled by a powermeasurement. The power of the resultant signal may be used to determinea scaling factor, taking into consideration processing speeds ofsubsequent digital processing blocks, maximum allowable signal level,etc. For example, the power measurement unit 140 can determine the powerof the compensated data signal. The resultant signal can then be scaled,e.g., by the digital scalar 142, by the determined scaling factor. Fromblock 218, the flow ends.

At block 220, the size of the growing box filter is increased. The flow200 moves from block 212 to block 220 after it is determined that thepower of the previous spur estimate is less than the power of thecurrent spur estimate. The length of the growing box filter can dependon the accuracy of the spur estimate. An increase in spur power canindicate that the growing box filter is centered on the spur frequency.An increase in spur power can also indicate that the growing box filteris accurately removing parts of the data signal that are not the spur,without removing the spur itself. In some instances, for computationaland implementation advantages, the size of the growing box filter may beincreased every 2^(n) samples, where n is a positive integer. When thespur frequency estimate is accurate, the size of the growing box filteris limited only by the size of the data (i.e. payload) field in thereceived signal. In some implementations, the size of the growing boxfilter may be increased until only the shifted spur signal remains. Fromblock 220, the flow continues at block 202.

It should be understood that the depicted flow diagram (FIG. 2) is anexample meant to aid in understanding embodiments and should not be usedto limit embodiments or limit scope of the claims. Embodiments mayperform additional operations, fewer operations, operations in adifferent order, operations in parallel, and some operationsdifferently. For example, although techniques for estimating andlimiting the size of the growing box filter using power comparison aredescribed, other techniques may also be used to estimate the size of thegrowing box filter. In some instances, the accuracy of the spur estimate(at the output of the filter) may be determined by determining thevariance of the spur estimate. In some implementations, the growing boxfilter (or any suitable unit) may comprise a look-up table to determinethe maximum length of the growing box filter. The length of the growingbox filter can depend on the accuracy of the spur estimate and can bedetermined theoretically or via simulations. Functionality can beimplemented to determine a spur estimate, calculate the variance of thespur estimate, and access the look-up table to determine the maximumlength of the growing box filter. Functionality can also be implementedto stop computing the spur estimate when the maximum length of thegrowing box filter is reached.

FIG. 3 is an example conceptual diagram illustrating the effect of thesize of the growing box filter and accuracy of the spur estimate on spurmitigation. In FIG. 3, graph 302 depicts the frequency response of thespur with a spur center frequency f_(spur). Graph 304 depicts thegrowing box filter's frequency response also centered on the spurfrequency f_(spur). Because the growing box filter begins as a filterwith a short pre-defined length, the growing box filter's frequencyresponse is wide. As shown in graph 306, the growing box filter'sfrequency response covers a wide range of frequencies. Therefore, asindicated in stage A, the growing box filter filters out a part of thedata signal along with the spur. This can lead to a loss of informationand incorrect decoding. At stage B, the length of the growing box filteris increased. This leads to a narrowing of the filter's frequencyresponse indicated in graph 312. As indicated in graphs 312 and 314, thegrowing box filter is narrow and filter's frequency response is centeredon the spur frequency f_(spur)). Therefore, at stage C, the growing boxfilter removes the data signal without affecting the spur. Thus, atstage C, an accurate estimate of the spur can be obtained. Graphs 320,322, and 324 depict the effect of an inaccurate spur estimate coupledwith a narrow filter response. In graph 320, the spur frequency estimate(f_(spur)) is incorrect. The actual spur frequency is represented byf_(spur) _(—) _(acc). This inaccuracy may be caused by inaccuratecomputations, digital errors, etc. In graph 322, the growing boxfilter's frequency response is narrow and the filter's frequencyresponse is centered on the inaccurate estimate of the spur frequency(f_(spur)). As indicated at stage D, in graph 324, the narrowness of thegrowing box filter response and an inaccurate spur frequency estimatelead to a filter-spur mismatch. Therefore, the filter does not capturethe spur completely. This can lead to an inaccurate spur estimate,erroneous decoding, and a reduction in receiver performance.

FIG. 4 is a flow diagram illustrating example operations for AGC controlof spur mitigation in packet detection. The flow 400 begins at block402.

At block 402, a received signal's amplitude is determined. The receivedsignal may be an incoming signal from a receive antenna. In someinstances, the received signal may be acquired from an amplifying unitor an ADC. The amplitude of the received signal can be determined usingan envelope detector. In some implementations, the received signal'spower may be computed and utilized for subsequent operations in flow400. From block 402, the flow continues at block 404.

At block 404, it is determined, e.g., by the AGC 108, whether thereceived signal's amplitude is within a specified range. The specifiedrange can be pre-determined and can be based on a maximum bit width ofthe ADC. In some implementations, to prevent continuous gain changes,the received signal may be monitored and scaled only if the receivedsignal's amplitude increases or decreases beyond the specified range.For instance, the signal may not be scaled if the difference between acurrent measurement of the received signal's amplitude and a previousmeasurement of the received signal's amplitude is within, e.g., 3% ofthe previous measurement of the received signal's amplitude. If it isdetermined that the received signal amplitude is outside the specifiedrange, the flow continues at block 406. Otherwise, the flow continues atbock 414.

At block 406, it is determined, e.g., by the AGC 108, whether thereceived signal amplitude is greater than the specified range. In someimplementations, it may be determined whether the received signal isgreater than an upper bound on the range by a threshold value, e.g., 10%of the upper bound on the range. In other implementations, the upperbound on the range may be used as a threshold. The threshold may bebased on the maximum bit width of the ADC, processing capacities ofsubsequent processing units in the receiver chain, maximum signallevels, etc. The threshold may be a pre-determined absolute value, apercentage of the maximum signal level, etc. If it is determined thatthe received signal amplitude is greater than the range, the flowcontinues at block 408. Otherwise, the flow continues at block 410.

At block 408, the gain of the variable gain amplifier (VGA) isincreased. For example, the AGC 108 may direct the VGA 104 to increasethe gain applied to the received signal. Increasing the gain of the VGAand thus increasing the received signal's strength can ensure that theoutput of the ADC optimally uses its available bit-width. For example,before the gain of the VGA is increased, the ADC may use only half ofits quantization levels to represent a very weak received signal, thusnot optimally representing the signal. After the gain of the VGA isincreased, the flow continues at block 412.

At block 412, taps of a moving average filter are reset, e.g., by theAGC 108. The moving average filter, e.g., moving average filter 116, isused to estimate the spur in the packet detection path. The movingaverage filter estimates the spur by filtering out parts of the receivedsignal that are not the spur. Once the taps of the moving average filterare set, they are typically used throughout the operation of the filter.A change in received signal amplitude can indicate a presence of adifferent spur, with a different amplitude, frequency, and phase.Clearing the moving average filter's taps every time the VGA's gain ischanged can ensure that the old taps do not affect the estimation of thenew spur signal. Operations of the moving average filter are furtherdescribed by the flow diagram in FIG. 5. From block 412, the flowcontinues at block 402.

At block 410, the gain of the variable gain amplifier is decreased. Forexample, the AGC 108 may direct the VGA 104 to decrease the gain appliedto the received signal. The flow 400 moves from block 406 to block 410,after it is determined that the received signal's amplitude is less thanthe specified range. In some implementations, it may also be determined,e.g., by the AGC 108, whether the received signal is less than a lowerrange by a threshold value, e.g., 10% of the lower range bound. In otherimplementations, the lower range bound may be used as a threshold.Decreasing the gain of the VGA and thus decreasing the received signal'sstrength can ensure that the ADC is not saturated by the presence of astrong spur in the received signal and that the signal is notmisrepresented. In other words, the received signal may be scaled to fitany spur in the received signal. From block 410, the flow continues atblock 412.

At block 414, the received signal is provided to a spur mitigation unitfor packet detection. The flow 400 moves from block 404 to block 414,after it is determined that the received signal amplitude is within thespecified range, thus precluding the need for gain modifications. Thespur mitigation unit for packet detection comprises functionality forremoving the spurs from the received signal. Operations for spurmitigation in packet detection are further described in FIG. 5. Fromblock 414, the flow ends.

FIG. 5 is a flow diagram illustrating example operations for spurmitigation in packet detection. The flow 500 begins at block 502.

At block 502, a signal is received, e.g., by the spur mitigation unitfor packet detection 160 of FIG. 1, after AGC processing. In someimplementations, the signal may be processed by one of more otherprocessing units (e.g., ADC unit, filter units, etc.) after AGCprocessing and before being received by the spur mitigation unit forpacket detection. The flow continues at block 504.

At block 504, a spur in the received signal is shifted to DC (0 Hz),e.g., via multiplier 114. Spurs typically occur at harmonics of adigital clock frequency. In some instances, a frequency generator (e.g.,local oscillator) may generate the spurs. Shifting the spur frequency toDC (0 Hz) allows for easier estimation of spur amplitude. Shifting thespur frequency to DC also allows a low-pass filter to filter out allparts of the data signal that are not the spur. The spur in the receivedsignal can be shifted to DC by multiplying the data signal by phaseslope representing a conjugate of the spur frequency (i.e., e^(−j2πf)^(spur) ^(t)) as described in Eq. 1. The flow continues at block 506.

At block 506, a spur estimate is calculated using a moving averagefilter. For example, the moving average filter 116 can calculate thespur estimate. A moving average filter is a finite impulse response lowpass filter. Unlike the growing box filter, the moving average filterdoes not process an increasing number of samples at every iteration. Inother words, the moving average filter has a pre-determined length. Thelength of the moving average filter may be determined by a frequency atwhich the received signal is sampled, the speed of subsequent processingunits, etc. At any instant, an N-tap moving average filter uses the mostrecent set of N sample values. The moving average output can berepresented by Eq. 5, where, y_(i)(moving average) and x_(i) are samplesof the moving average output and shifted signal with the spur at DC(determined at block 504), respectively.

$\begin{matrix}{{{yi}\left( {{moving}\mspace{14mu}{average}} \right)} = \frac{\sum\limits_{k = 0}^{N - 1}x_{i - k}}{N}} & \left( {{Eq}.\mspace{11mu} 5} \right)\end{matrix}$The moving average filter removes parts of the received signal that arenot the spur. Thus, the output of the moving average filter is a spurestimate (amplitude and phase). The flow continues at block 508.

At block 508, the spur estimate is shifted to the original spurfrequency, e.g., via multiplier 118. At block 504, the spur was shiftedto DC to enable the moving average filter to determine a best estimateof the spur. To remove the spur from the received signal, the spurestimate should be accurately represented in the frequency domain andtherefore, should be shifted to its original spur frequency. This isdone, as depicted in Eq. 3, by multiplying the spur estimate from themoving average filter by a phase slope representing the spur frequencyf_(spur) (i.e., e^(j2πf) ^(spur) ^(t)). The flow continues at block 510.

At block 510, the shifted spur estimate (determined at block 508) issubtracted from the acquired received signal (received at block 502),e.g., via adder 120. This is represented in Eq. 4. The flow continues atblock 512.

At block 512, the compensated received signal is provided to subsequentpacket processing units. Subsequent packet processing units can includea CCK demodulator, a timing and frequency estimation and correctionunit, a channel estimation unit, etc. In some instances, the compensatedreceived signal may also be scaled to optimally represent the signalwithout the spur. From block 512, the flow ends.

It should be understood that the depicted flow diagrams (FIGS. 2, 4, and5) are examples meant to aid in understanding embodiments and should notbe used to limit embodiments or limit scope of the claims. Embodimentsmay perform additional operations, fewer operations, operations in adifferent order, operations in parallel, and some operationsdifferently. For example, in some implementations, the spur frequency isassumed to be known, while in other implementations, the spur frequencycan be estimated. The spur frequency can be estimated by transmitting anempty packet (i.e., a packet without a payload), via a transmitter andreceiving this packet at the receiver. At the receiver, a Fast FourierTransform (FFT) operation can be performed on the received empty packetfollowed by a spectral analysis of the FFT results. In the FFT spectrum,spurs are represented as spikes and depending on the accuracy of the FFToperation, an accurate estimate of the spur frequency can be obtained.

Also, in some implementations, a combination of a finite impulseresponse (FIR) filter and an infinite impulse response (IIR) filter(“combination FIR/IIR filter”) may be used in place of the growing boxfilter 132 of FIG. 1. In some implementations, the frequency of the spur(e.g., digital clock) may be different from a local oscillator frequency(i.e. F1 and F2) used to shift the spur for spur mitigation operations.The combination FIR/IIR filter comprises an IIR filter, which takes aninitial value from the FIR filter. Using an IIR filter can ensure awider pass band as compared to a growing box filter. Using an initialvalue from the FIR filter can decrease the converging time of the IIRfilter. The combination FIR/IIR filter can provide better performance ascompared to the growing box filter if the estimated spur frequency(e.g., F1) is not accurate. For example, if the FIR filter is a 32 tapfilter, the IIR filter starts at n=33. Eq. 6 represents the initialvalue of the IIR filter. Eq. 7a represents a first order IIR filter. Thefirst output sample of the FIR filter at n=33 (y(33)) can be representedby Eq. 7b.

In Eq. 6, Eq. 7a, and Eq. 7b, x(n) represents the n^(th) sample of datato be filtered, y_(FIR) represents the output of the FIR filter, and yrepresents the output of the IIR filter. The weight a_(n-1) applied tothe previous IIR sample is a parameter, which may be used to configure afilter bandwidth. The value of a_(n-1) is configurable and may depend onthe desired convergence speed and noise level.

$\begin{matrix}{{y_{FIR}(32)} = \frac{{x(1)} + {x(2)} + \ldots + {x(32)}}{32}} & {{Eq}.\mspace{11mu} 6} \\{{{{y(n)} = {{x(n)} + {a_{n - 1}*{y\left( {n - 1} \right)}}}};{n = 33}},\ldots} & {{{Eq}.\mspace{14mu} 7}a} \\{{y(33)} = {{x(33)} + {a_{n - 1}*{y_{FIR}(32)}}}} & {{{Eq}.\mspace{14mu} 7}b}\end{matrix}$

Embodiments may take the form of an entirely hardware embodiment, anentirely software embodiment (including firmware, resident software,micro-code, etc.) or an embodiment combining software and hardwareaspects that may all generally be referred to herein as a “circuit,”“module” or “system”. Furthermore, embodiments of the inventive subjectmatter may take the form of a computer program product embodied in anytangible medium of expression having computer usable program codeembodied in the medium. The described embodiments may be provided as acomputer program product, or software, that may include amachine-readable medium having stored thereon instructions, which may beused to program a computer system (or other electronic device(s)) toperform a process according to embodiments, whether presently describedor not, since every conceivable variation is not enumerated herein. Amachine-readable medium includes any mechanism for storing ortransmitting information in a form (e.g., software, processingapplication) readable by a machine (e.g., a computer). Themachine-readable medium may include, but is not limited to, magneticstorage medium (e.g., floppy diskette); optical storage medium (e.g.,CD-ROM); magneto-optical storage medium; read only memory (ROM); randomaccess memory (RAM); erasable programmable memory (e.g., EPROM andEEPROM); flash memory; or other types of medium suitable for storingelectronic instructions. In addition, embodiments may be embodied in anelectrical, optical, acoustical or other form of propagated signal(e.g., carrier waves, infrared signals, digital signals, etc.), orwireline, wireless, or other communications medium.

Computer program code for carrying out operations of the embodiments maybe written in any combination of one or more programming languages,including an object oriented programming language such as Java,Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The program code may execute entirely on a user's computer,partly on the user's computer, as a stand-alone software package, partlyon the user's computer and partly on a remote computer or entirely onthe remote computer or server. In the latter scenario, the remotecomputer may be connected to the user's computer through any type ofnetwork, including a local area network (LAN), a personal area network(PAN), or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider).

FIG. 6 depicts an example wireless device including a mechanism forimplementing spur mitigation. In one implementation, the wireless devicemay be a WLAN device. The WLAN device includes a processor unit 602(possibly including multiple processors, multiple cores, multiple nodes,and/or implementing multi-threading, etc.). The WLAN device includes amemory unit 606. The memory unit 606 may be system memory (e.g., one ormore of cache, SRAM, DRAM, zero capacitor RAM, Twin Transistor RAM,eDRAM, EDO RAM, DDR RAM, EEPROM, NRAM, RRAM, SONOS, PRAM, etc.) or anyone or more of the above already described possible realizations ofmachine-readable media. The WLAN device also includes a bus 610 (e.g.,PCI, ISA, PCI-Express, HyperTransport®, InfiniBand®, NuBus, etc.), andnetwork interfaces 604 that include at least one wireless networkinterface (e.g., a WLAN interface, a Bluetooth® interface, a WiMAXinterface, a ZigBee® interface, a Wireless USB interface, etc.). TheWLAN device also includes a receiver unit 608. The receiver unit 608comprises a packet and a data processing unit each having spurmitigation functionalities described in accordance with FIGS. 1-5. Thereceiver unit 608 also comprises an automatic gain control (AGC) unitand a digital scalar, which controls the gain of the received signal andensures that the signal optimally utilizes the available bit-widths.Additionally, the receiver unit 608 can also comprise one or more otherprocessing units including a Fast Fourier Transform unit (FFT), analogto digital converter (ADC), one or more filter and amplifier stages, adecoding unit, etc. Any one of the above-described functionalities maybe partially (or entirely) implemented in hardware and/or on theprocessing unit 602. For example, the functionality may be implementedwith an application specific integrated circuit, in logic implemented inthe processing unit 602, in a co-processor on a peripheral device orcard, etc. Further, realizations may include fewer or additionalcomponents not illustrated in FIG. 6 (e.g., additional networkinterfaces, peripheral devices, etc.). The processor unit 602 and thenetwork interfaces 604 are coupled to the bus 610. Although illustratedas being coupled to the bus 610, the memory 606 may be coupled to theprocessor unit 602.

While the embodiments are described with reference to variousimplementations and exploitations, it will be understood that theseembodiments are illustrative and that the scope of the inventive subjectmatter is not limited to them. In general, techniques for spurmitigation as described herein may be implemented with facilitiesconsistent with any hardware system or hardware systems. Manyvariations, modifications, additions, and improvements are possible.

Plural instances may be provided for components, operations, orstructures described herein as a single instance. Finally, boundariesbetween various components, operations, and data stores are somewhatarbitrary, and particular operations are illustrated in the context ofspecific illustrative configurations. Other allocations of functionalityare envisioned and may fall within the scope of the inventive subjectmatter. In general, structures and functionality presented as separatecomponents in the exemplary configurations may be implemented as acombined structure or component. Similarly, structures and functionalitypresented as a single component may be implemented as separatecomponents. These and other variations, modifications, additions, andimprovements may fall within the scope of the inventive subject matter.

What is claimed is:
 1. A receiver for a wireless communication device,the receiver comprising: an analog-to-digital converter (ADC) operableto convert an RF signal received at the receiver to a digital signal;and a spur mitigation unit operable to receive an output signal of theADC, wherein the spur mitigation unit comprises: a growing box filteroperable to determine a current estimate of a spur signal of the ADCoutput signal, and a power measurement unit operable to: determine apower of the current estimate of the spur signal and a power of aprevious estimate of the spur signal, determine whether the power of theprevious estimate of the spur signal is greater than the power of thecurrent estimate of the spur signal; wherein, if the power of theprevious spur signal estimate is greater than the power of the currentspur signal estimate, the spur mitigation unit is operable to remove thespur signal from the ADC output signal based on the previous spur signalestimate.
 2. The receiver of claim 1, wherein, if the power of theprevious spur estimate is less than the power of the current spurestimate, the power measurement unit is operable to increase a size ofthe growing box filter, wherein, after increasing the size of thegrowing box filter, the growing box filter is operable to determine anew estimate of the spur signal of the ADC output signal.
 3. Thereceiver of claim 2, wherein the spur mitigation unit is operable to:terminate execution of spur mitigation operations in response todetermining that the size of the growing box filter is greater than amaximum size of the growing box filter.
 4. The receiver of claim 2,wherein the power measurement unit is operable to: determine a varianceassociated with the current spur estimate; and access a look-up table todetermine a maximum length of the growing box filter based, at least inpart, on the variance associated with the current spur estimate.
 5. Thereceiver of claim 2, wherein the power measurement unit is operable to:determine a variance associated with the current spur estimate; anddynamically determine and vary the size of the growing box filter based,at least in part on the variance associated with the current spurestimate.
 6. The receiver of claim 1, wherein the power measurement unitoperable to determine whether the power of the previous estimate of thespur signal is greater than the power of the current estimate of thespur signal comprises the power measurement unit operable to: determinewhether a difference between the power of the previous estimate of thespur signal and the power of the current estimate of the spur signal isgreater than a predetermined threshold; determine whether the power ofthe previous estimate of the spur signal is greater than the power ofthe current estimate of the spur signal by a predetermined percentage ofthe power of the previous estimate of the spur signal; or determinewhether the power of the current estimate of the spur signal is within apredetermined percentage of the previous estimate of the spur signal. 7.The receiver of claim 1, wherein, if the power of the previous spursignal estimate is greater than the power of the current spur signalestimate, the spur mitigation unit operable to remove the spur signalfrom the ADC output signal based on the previous spur signal estimatecomprises the spur mitigation unit operable to: shift the previous spursignal estimate to an initial spur frequency estimate that is differentfrom a current spur frequency associated with the previous spur signalestimate; subtract the previous spur signal estimate centered at theinitial spur frequency estimate from the output signal of the ADC toyield a compensated digital signal; and provide the compensated digitalsignal to a digital scalar that is operable to scale the compensateddigital signal by a scaling factor, wherein the scaling factor isdetermined based, at least in part, on a power associated with thecompensated digital signal and a maximum allowable power level.
 8. Amethod for implementing spur mitigation in a receiver of a wirelesscommunication device, the method comprising: converting an RF signalreceived at the receiver of the wireless communication device into adigital signal using an analog-to-digital converter (ADC); providing anoutput signal of the ADC to a spur mitigation unit in a packet detectioncircuit of the receiver; at the spur mitigation unit, determining acurrent estimate of a spur signal of the ADC output signal using agrowing box filter; determining a power of the current estimate of thespur signal and a power of a previous estimate of the spur signal;determining whether the power of the previous estimate of the spursignal is greater than the power of the current estimate of the spursignal; and if it is determined that the power of the previous spursignal estimate is greater than the power of the current spur signalestimate, removing the spur signal from the ADC output signal based onthe previous spur signal estimate.
 9. The method of claim 8, wherein, ifit is determined that the power of the previous spur estimate is lessthan the power of the current spur estimate, the method furthercomprises: determining a variance associated with the current spurestimate; determining a maximum size of the growing box filter based, atleast in part on the variance associated with the current spur estimate;determining whether increasing the size of the growing box filter willcause the growing box filter to exceed the maximum size of the growingbox filter; in response to determining that increasing the size of thegrowing box filter will cause the growing box filter to exceed themaximum size of the growing box filter, terminating execution of spurmitigation operations; and in response to determining that increasingthe size of the growing box filter will not cause the growing box filterto exceed the maximum size of the growing box filter, increasing a sizeof the growing box filter; and determining a new estimate of the spursignal of the ADC output signal after said increasing the size of thegrowing box filter.
 10. The method of claim 8, wherein said determiningwhether the power of the previous estimate of the spur signal is greaterthan the power of the current estimate of the spur signal comprises oneof: determining whether a difference between the power of the previousestimate of the spur signal and the power of the current estimate of thespur signal is greater than a predetermined threshold; determiningwhether the power of the previous estimate of the spur signal is greaterthan the power of the current estimate of the spur signal by apredetermined percentage of the power of the previous estimate of thespur signal; or determining whether the power of the current estimate ofthe spur signal is within a predetermined percentage of the previousestimate of the spur signal.
 11. The method of claim 8, wherein saidremoving the spur signal from the ADC output signal based on theprevious spur signal estimate comprises: shifting the previous spursignal estimate to an initial spur frequency estimate that is differentfrom a current spur frequency associated with the previous spur signalestimate; subtracting the previous spur signal estimate centered at theinitial spur frequency estimate from the output signal of the ADC toyield a compensated digital signal; and scaling the compensated digitalsignal by a scaling factor, wherein the scaling factor is determinedbased, at least in part, on a power associated with the compensateddigital signal and a maximum allowable power level.
 12. A receiver for awireless communication device, the receiver comprising: a variable gainamplifier (VGA) operable to change an amplitude of an RF signal receivedat the receiver; an analog-to-digital converter (ADC) coupled to the VGAand operable to convert an output signal of the VGA into a digitalsignal; an automatic gain control (AGC) unit coupled to the ADC andoperable to determine whether an amplitude of an output signal of theADC is within a predetermined amplitude range; wherein, if the amplitudeof the output signal of the ADC is outside the predetermined amplituderange, the AGC unit is operable to modify a gain associated with the VGAsuch that the ADC output signal has an amplitude within thepredetermined amplitude rage; and a first spur mitigation unit forpacket detection operable to determine an estimate of a spur signal ofthe ADC output signal and remove the spur signal from the ADC outputsignal based on the estimate of the spur signal, wherein the first spurmitigation unit comprises a moving average filter operable to generatethe estimate of the spur signal.
 13. The receiver of claim 12, whereinthe first spur mitigation unit is operable to: determine the estimate ofthe spur signal of the ADC output signal; and remove the spur signalfrom the ADC output signal based on the estimate of the spur signal whenthe amplitude of the ADC output signal is within the predeterminedamplitude range.
 14. The receiver of claim 12, wherein the AGC unit isfurther operable to: determine whether an amplitude of the output signalof the ADC is within a predetermined amplitude range; in response todetermining that the amplitude of the output signal of the ADC is withinthe predetermined amplitude range, provide the output signal of the ADCto the first spur mitigation unit for executing one or more spurmitigation operations to remove the spur signal from the output signalof the ADC. in response to determining that the amplitude of the outputsignal of the ADC is not within the predetermined amplitude range,modify a gain associated with the VGA such that the ADC output signalhas an amplitude within the predetermined amplitude range; and reset atleast the moving average filter of the first spur mitigation unit inresponse to the AGC unit modifying the gain associated with the VGA. 15.The receiver of claim 14, wherein in response to determining that theamplitude of the output signal of the ADC is not within thepredetermined amplitude range, the AGC unit operable to modify the gainassociated with the VGA such that the ADC output signal has an amplitudewithin the predetermined amplitude range comprises the AGC unit operableto: increase the gain associated with the VGA such that the ADC outputsignal has an amplitude within the predetermined amplitude range, if theamplitude of the ADC output signal is greater than the predeterminedamplitude range; and decrease the gain associated with the VGA such thatthe ADC output signal has an amplitude within the predeterminedamplitude range, if the amplitude of the ADC output signal is less thanthe predetermined amplitude range.
 16. The receiver of claim 12, furthercomprising a second spur mitigation unit for data processing operable todetermine an estimate of a spur signal of the ADC output signal andremove the spur signal from the ADC output signal based on the estimateof the spur signal, wherein the second spur mitigation unit comprises agrowing box filter operable to generate the estimate of the spur signal.17. The receiver of claim 16, wherein the second spur mitigation unit isoperable to: determine a current estimate of a spur signal of the ADCoutput signal using the growing box filter; determine whether a power ofa previous estimate of the spur signal is greater than the power of thecurrent estimate of the spur signal; if it is determined that the powerof the previous spur signal estimate is greater than the power of thecurrent spur signal estimate, remove the spur signal from the ADC outputsignal received at the second spur mitigation unit based on the previousspur signal estimate if it is determined that the power of the previousspur signal estimate is less than the power of the current spur signalestimate, increase the size of the growing box filter, and determine anew estimate of the spur signal of the ADC output signal.
 18. Thereceiver of claim 16, further comprising a matched root-raised cosine(RRC) filter coupled to the ADC, wherein the matched RRC filter isoperable to filter the ADC output signal and provide the ADC outputsignal to the first spur mitigation unit and a second spur mitigationunit.
 19. The receiver of claim 16, wherein the second spur mitigationunit is further operable to: determine a power of an output signal ofthe second spur mitigation unit; determine a scaling factor based, atleast in part, on the power of the output signal of the second spurmitigation unit; and scale the output signal of the second spurmitigation unit using the scaling factor.
 20. The receiver of claim 12,operable to process RF signals modulated using a complementary codekeying (CCK) modulation technique.